Tandem drive axles for use in heavy-duty vehicles such as trucks normally employ a gear-type power divider for transmitting driving torque to each of the drive axles. This power divider often includes an interaxle differential unit for providing a differential relationship between the two driving axles under normal operating conditions. Provision is also made to "lock out" this differential unit under conditions when the differential action would cause a loss of traction. The power divider is also conventionally provided with an auxiliary transmission unit at the input thereof to provide one or more additional gear ratios, and more specifically an additional low speed ratio which is desirably used during off-highway operations.
As illustrated in U.S. Pat. No. 3,611,832, it has been conventional to construct and operate the auxiliary transmission independently of the interaxle differential, with the auxiliary transmission being of substantially conventional construction wherein a pair of shafts are interconnected by several gear pairs. These known systems have operated in a satisfactory manner but, as is obvious to those familiar with this technology, there is a continuing effort to improve upon such geared systems. In particular, since geared systems of this type as applied to heavy-duty vehicles are large and mechanically complex, it is obviously desirable to simplify such systems, whenever possible, without detracting from the desired mode of operation. Any simplifications which reduce the number of components, the weight or size of the overall system, or which sufficiently simplify the system so as to hopefully reduce the required maintenance and repair, while at the same time retaining or improving upon the desired mode of operation, is thus a strongly pursued objective.
Accordingly, it is an object of the present invention to provide an improved geared drive system and, in particular, an improved auxiliary transmission for use with the power divider of a tandem axle construction.
Another object of this invention is to provide an improved auxiliary transmission, as aforesaid, for use with a power divider which may include an interaxle differential, with the auxiliary transmission being of simplified structure and reduced size while at the same time providing the power divider with the desired speed ratios.
Still another object of the invention is to provide an improved auxiliary transmission for use with an interaxle power divider, as aforesaid, wherein the auxiliary transmission permits the power divider to be operated at two different speed ratios while at the same time the auxiliary transmission possesses only a single pair of meshing gears, whereby the size and structure of the auxiliary transmission and particularly the number of gears is greatly reduced.
A further object is to provide an auxiliary transmission, as aforesaid, which utilizes a pair of gears associated with the power divider to permit accomplishment of the desired low speed ratio which is achieved through use of the auxiliary transmission, whereby said pair of gears perform a dual function in that they comprise an essential part of both the power divider and the auxiliary transmission.
The objects and purposes of this invention, including those mentioned above, have been met by connecting an auxiliary transmission unit to the input of a power divider, which power divider is connected between a pair of pinion shafts for respectively driving the front and rear axles of a tandem axle construction. The power divider includes at least one pair of meshing axle gears drivingly interconnected between the pinion shafts for transmitting torque to each of the axles. The auxiliary transmission unit includes a single pair of meshing transmission gears, one thereof being associated with the transmission input shaft and the other being associated with a parallel countershaft. The output of the transmission unit comprises one of the shafts of the power divider. A clutch sleeve is shiftably associated with the input shaft and can be engaged with the transmission output for permitting direct driving thereof, or it can alternately be shifted into a disengaged position whereby the input transmission gear drivingly rotates the countershaft to thereby provide a reduced speed ratio through the auxiliary transmission unit. The countershaft is drivingly connectable to one of the axle gears for driving the power divider when the auxiliary transmission is in its low speed condition.
Other objects and advantages of the invention will be apparent to persons familiar with systems of this type upon reading the following specification and inspecting the accompanying drawings.